The present invention relates to detection of internal faults on power system transmission lines by phase angle comparisons at ends of the transmission lines. Phase comparison relaying is a type of differential relaying that compares the phase angles of the currents entering one terminal of a transmission line with the phase angles of the currents entering all the remote terminals of the same line. For internal faults, the currents entering all the terminals will be in phase, and for conditions of an external or through fault, or for normal load flow, the currents entering any one terminal will be 180 degrees out of phase with the currents entering at least one of the remote terminals. The phase comparison relay scheme makes this phase angle comparison and trips the associated breakers for internal faults. Because terminals of a transmission line are normally many miles apart, a communication channel is needed between the terminals to make this comparison.
To compare each of three phase currents individually, separate communication channels would be required. To reduce the number of required channels to one, all three phase currents can be mixed to produce a single phase quantity whose magnitude and phase angle represent the magnitude and phase angle of the three original currents. This mixed single phase quantity can be compared with a similarly obtained quantity at the remote end or ends of the line.
In many techniques, fault detectors are provided to initiate phase comparisons only when a fault occurs on or in the general vicinity of the protected line. In conventional phase comparison methods, phase angle information is exchanged between ends of a transmission line in the form of a two level (Boolean) signal or three level signal that changes level when a monitored current crosses a certain threshold (a fault occurs). The level of a two level channel is changed whenever the current crosses a single threshold. For a three level channel, the channel level is controlled by the current level which may be within one of three ranges. Conventional phase comparison methods generally have sensitivity and security limitations because current measurements are subject to uncertainty.
In one technique, a microprocessor at each terminal compares the coincident time of a positive half cycle of mixed single phase current from its terminal when a fault is present (when the current exceeds a high fault detection threshold FDH) with an absence of an output signal from its receiver. If a fault occurs at a terminal and the receiver does not produce an output signal for a specified period of time, during the positive half cycle of single phase mixed current, a trip output signal will be obtained.
Whether the receiver provides an output signal depends on the keying of the remote transmitter. Remote transmitters are keyed ON by a signal from a FDL (low fault detection threshold) comparison if the current exceeds FDL and keyed OFF if FDL is not exceeded and/or if positive half cycles of current are occurring. Thus a blocking signal is initiated by an associated transmitter and sent to a remote receiver to produce an output signal to block tripping during external faults during the negative half cycle when the mixed single phase current exceeds the low fault detection threshold.
For an internal fault, the currents entering both ends of the line are in phase. Thus, during the half cycle that a remote mixing network is providing a positive current and the receiver at a local transmitter is producing no output signal, tripping will take place a both ends of the line if FDH is exceeded at the local transmitter. For an external fault, the current entering one terminal is 180 degrees out of phase with the current entering the other terminal. During the half cycles when the remote mixer is providing a positive current and FDH is exceeded (so that the associated receiver at the local transmitter is providing an output signal), no tripping will take place.